Was working fine last summer, but isn't blowing cold at all. Compressor is engaging, but no cool air. I've added freon, but still nothing. What is the likely culprit and how do I check it?

Check the POA valve as freon may not be flowing, the expansion valve or the orifice tube could be plugged.

Most likely the orifice tube.

No orifice tube in a first gen A/C system, it uses an expansion valve with a temperature bulb.

Likely problem is a loss, or complete lack of refrigerant. The compressor clutch on a first gen A/C system will engage as long as there is power to the circuit, and the air temperatrue going into the evaporator is greater than 29 degrees farenheit. They do not have a low refrigerant pressure cut out like the newer systems do, they will run completely dry if you put the switch to AC. You need to recharge the system until the outlet pipe of the evaporator gets cold. Of couse you need to find and fix any leaks you may have, and need to draw a vacuum in the system prior to adding any new refrigerant or you will never get any satisfactory performance out of the system.

Mark, with a AC pressure gauge, can't I check to see if there is freon in the system? By the way I'm running the 134.

Yes you can use a gauge set, but you have to know the inlet and outlet temperatures of the compressor, as well. Compressor head pressures betwen the compressor and the condenser) can be as high as 340 psi so be careful.

The discharge of the compressor should be hot (around 150 to 180 degrees) and the suction side should be cold (less than 30 degrees) if there is enough refrigerant. These numbers are affected by the air temperature going into the condenser.

The factory test consists of running the A/C system on MAX and running the engine at 2000 RPM. After 1 minute the minimum drop in temperature from the center duct based on the condeser inlet temperature is:

70 degrees 20 degree drop
80 degrees 25 degree drop
90-110 degrees 30 degree drop

R134a as the only substitute for R12 refrigerant that GM recommends. Recharge the system with about 15% less R134a than the R12 specification. Their formula is: (R12 lbs x .9) - .25 lb. = R134a Charge

Full charge with R134A in a stock system (3.75 Lbs) is around 48 to 50 oz of refrigerant.

Here's a link: This is for newer A/C systems operation. Remember the there are no High pressure switches, or low pressure cutouts on early Camaro systems.

http://www.delanet.com/~pparish/ac-dg.htm

tell me what your low side and high side readings are on your manifold gauge set
and the temp outside were you are checking and i will diagnose for you
with a/c on max eng running of course